In the construction of capacitors, such as power factor correction capacitors, the capacitor packs are formed of alternate layers of metal foil and a solid dielectric material such as paper or polymeric film, which is impregnated with a liquid dielectric. In the past, polychlorinated biphenyl has seen wide use as a dielectric fluid for impregnating the paper or polymeric film dielectric material. While the polychlorinated biphenyls, such as for example, trichlorodiphenyl, produce an effective dielectric system for a capacitor, their usage has provided certain ecology problems in that the polychlorinated biphenyls are virtually non-biodegradable, with the result that if leakage or rupture occurs in the capacitor casing, or if the capacitor is discarded as obsolete, the polychlorinated biphenyl will remain as a pollutant in the environment and will not degrade to any appreciable extent, even over extended periods of years.
As a consequence of the pollution problem presented by the polychlorinated biphenyls, a number of dielectric liquids have more recently been used as replacements for polychlorinated biphenyls, including such materials as isopropyl biphenyl, phthalate ester/trichlorobenzene, and buylated monochlorodiphenyl oxide.
Of these fluids, the phthalate ester/trichlorobenzene mixture has come under question because of its environmental acceptability. Trichlorobenzene, for example, has been shown to form, in the presence of sunlight, polychlorinated biphenyl, and thus there is an implication that the exposure of this compound to the environment may generate the undesirable polychlorinated biphenyls.
Isopropyl biphenyl performs satisfactorily from an electrical standpoint, but the flammability properties are below that of the polychlorinated biphenyls, being only equivalent to regular transformer oil in its flash and fire properties.